1. Field of the Invention
The present invention relates to the field of flat displaying, and in particular to an active organic electroluminescence device back panel and a manufacturing method thereof.
2. The Related Arts
A flat display device has various advantages, such as thin device body, low power consumption, and being free of radiation, and is thus of wide applications. The flat display devices that are currently available include liquid crystal displays (LCDs) and organic electroluminescence devices (OELDs), which are also referred to as organic light emitting diodes (OLEDs).
The known liquid crystal displays are generally backlighting liquid crystal displays, which include an enclosure, a liquid crystal display panel arranged in the enclosure, and a backlight module mounted inside the enclosure. The principle of operation of the liquid crystal display panel is that liquid crystal molecules are interposed between two parallel glass substrates and a driving voltage is applied to the glass substrates to control the rotation of the liquid crystal molecules so as to refract out the light from the backlight module to form an image.
The organic electroluminescence devices, which show the characteristics of self-illumination, high brightness, wide view angle, high contrast, flexibility, and low energy consumption, attract wide attention for serving as the next-generation display measures and gradually substitute the conventional liquid crystal displays for wide applications in various fields including mobile phone screens, computer monitors, and full-color television. The organic electroluminescence devices are different from the conventional liquid crystal displays in that they need no backlight and they use extremely thin coating layers of organic materials directly formed on the glass substrates so that when electrical currents flow therethrough, the organic material coating layers emit light.
The currently available organic electroluminescence devices are classified according to the driving methods used and include passive-matrix organic light emitting diodes (PMOLEDs) and active-matrix organic light emitting diodes (AMOLEDs). As shown in FIG. 1, the AMOLEDs generally comprise a substrate 502, a thin-film transistor (TFT) 504 formed on the substrate 502, and an organic light-emitting diode 506 formed on the TFT 504. The TFT 504 drives the organic light-emitting diode 506 to give off light in order to display a corresponding image.
Thin-film transistors include driving TFTs, switch TFTs, and other circuit TFTs. In the manufacture of an organic electroluminescence device, gate insulation (GI) layers of the driving TFTs, the switch TFTs, and other circuit TFTs are formed at the same time, having the same thickness, and this makes the driving TFTs, the switch TFTs, and the other circuit TFTs possess identical gate capacitance (Ci). In respect of sub-threshold swing (S.S.) of a thin-film transistor, it is known from the formula: S.S.=kT/q In10(1+Cd/Ci) that sub-threshold swing of a thin-film transistor is determined by the gate capacitance, which is in turn determined by the thickness of the gate insulation layer (C=εA/d) It is thus obvious that when the gate insulation layers of the driving TFTs, the switch TFTs, and the other circuit TFTs are of identical thicknesses, the sub-threshold swings of the driving TFTs, the switch TFTs, and the other circuit TFTs would be of the same magnitude.
The physical meaning of the sub-threshold swing of a thin-film transistor is the slope of the curve of gate voltage and drain current in a sub-threshold zone. Generally, curves that have very high slopes in the sub-threshold zone are disadvantageous for input control of a control voltage that reflects grey level variation. Curves having relatively low slopes in the sub-threshold zone are advantageous for control of the control voltage that reflects grey level variation. Thus, when the sub-threshold swing of a thin-film transistor is relatively small, it would be difficult to define the grey levels of an organic electroluminescence device. On the other hand, if the thickness of the gate insulation layer is increased to increase the sub-threshold swing of the driving TFTs for the purposes of easy define of the grey level, then the sub-threshold swings of the switch TFTs and other circuit TFTs would be also increased, leading to an increase of operating voltages thereby lowering the operation speeds of the circuits.
Thus, there is a conflict existing between the sub-threshold swing of the driving TFT and the sub-threshold swings of the switch TFTs and other circuit TFTs. It is necessary to have a solution that overcomes the conflict in order to further improve the quality of the organic electroluminescence devices.